The study of factors affecting culling and survival distribution functionin Iranian Holstein cows

Reducing of economic lifetime and culling of cow are costly and sophisticatedly process in animal industries (Dohoo et al. 2010). In dairy cattle herds, productive and economic lifetime of cow is important because of their substantial effects on pure benefit of every dairy farm. The productive lifetime is called the interval between calving and culling of cow (Vollema and Groen 1996; Sewalem et al. 2005). Nowadays lifespan is considerably noticed because of increasing of rate culling dairy cattle in most of herds. High culling of dairy cows lead-in few cows get to physical maturity stage and complete production (McCullough and DeLorenzo 1996).
Recognizing of most important factors affecting culling of dairy cows, longevity performance and economical lifetime and survival distribution function can use for making appropriate breeding strategies in order to improving of longevity and productive lifetime of Iranian Holstein dairy cow. So, the objective of this study was to investigate the culling reasons and factors affecting it and to determine survival distribution function in Iranian Holstein dairy cows.
Material and In This study, the records of lifespan and culling of 971428 Holstein dairy cows from 3872 herds during 1996 to 2013 were used. Data were collected by the Animal Breeding Center of Iran. Initial data were edited using R and FoxPro 2.6. In Iranian dairy cattle database, 55 reasons of culling were presented, so we categorized them to 12 general groups. For determination of frequencies of culling reasons, survival package of R program was used (Therneau 2015).
In this study, season, age at first calving, parity, lactation stage and milk yield as factors affecting culling (independent variables) were investigated. For analyzing of data logistic procedure of SAS 9.2 were used.
Lifetime of animals was evaluated as number of days from birth to culling or censoring of data. Cows that were not culled at the beginning of the study, as censored records considered and the zero code (0 censor code) was assigned to them. Also, culled animals that having culled code were considered as not censored (1 censor cod). In order to determine survival distribution function in Iranian Holstein dairy cow, lifetest procedure of SAS 9.2 were used. Survival distribution function was calculated by Kaplan-Meier equation (Kaplan and Meier 1958).
The results of the study showed that reproductive disorders were the most common reason of culling in Holstein dairy cows (23.61%). Reproductive disorders like dystocia, retained placenta and problems related to calving, infertility, repeated abortion and other reproductive disorders are the most typical reason of culling (Ansari-Lari et al. 2012; Aziz Zadeh 2011; Heravi Moussavi et al. 2007). In this study, the second reason of culling were mastitis and problems related to udder (18.18%). When milk production increases, problems related to mastitis, reproductive disorders and health of dairy cow will increase (Abe et al. 2009). The third factor affecting culling was non-economical cow (12.48%).
The results showed that considered factors (season, age at first calving, parity, lactation stage and milk yield) had significant effect on risk of culling in Holstein cows (P According to the high culling rate in herds, in order to increasing lifespan and longevity of Holstein cows it is necessary to improve environmental conditions and management practices and, to control and manage factors affecting culling, to prevent and treat the diseases. Also, planning of selection programs in dairy cows based on health, fertility and mastitis traits is recommendable.